Investigation of stress distribution within an endodontically treated tooth restored with different restorations.

Background/purpose: Recently, metal-free restoration has become the standard in prosthetic treatment. However, it is still unclear which combination is most effective in preventing root fracture and secondary caries. The purpose of this study was to evaluate the influence of different post systems, crown materials, crown thickness and luting agents on the stress distribution around the crown margins, cervical dentin and the tip of the post. Materials and methods: Ninety-six mandibular first premolar models were developed and analyzed using finite element analysis (FEA). Two designs of crowns, six kinds of crown materials, four types of post and core systems and two kinds of luting agents were included and evaluated for the stress distribution within the abutment teeth. The Von Mises stress magnitudes were compared among all models. Results: The stress at the tip of the post decreased as the young's modulus of luting agent decreased; The stress concentrated more at the cervical area (dentin and crown), as the physical properties of the crown material increased. Conclusion: Crowns fabricated using polyetheretherketone (PEEK) can reduce the stress concentration at the cervical area, so it may be possible to reduce the amount of tooth reduction during abutment tooth preparation. The stress distribution around the post tip is affected by the post and core systems and luting agent, regardless of crown materials and thickness. When inserting a post of the higher Young's modulus such as zirconia post, methyl methacrylate luting cement can reduce the stress concentration at the tip of the post.

The investigation of the stress distribution in abutment teeth for connected crowns.

BACKGROUND/PURPOSE: With the advancement of an over aging society, the average number of remaining teeth has increased. However, these remaining teeth do not always have sufficient alveolar bone support, and sometimes fabricated connected crowns are applied. This study evaluated the influence of crown material, crown thickness, and alveolar bone resorption on the stress distribution within the abutment teeth of connected crowns. MATERIALS AND METHODS: Using structural analysis software, a premolar crown model was fabricated. Three kinds of crown materials, two types of crown thickness, two types of post and core systems, and two levels of alveolar bone were assumed and evaluated for the stress distribution within the abutment teeth. RESULTS: The higher material properties crown was, the more stress was concentrated at the marginal area. The composite resin core showed larger stress values around the marginal area, and the metal core showed larger stress values at the tip of the post. Alveolar bone resorption progressed, the marginal area stress value increased. CONCLUSION: The low elastic modulus crown material polyetheretherketone (PEEK) prevented stress concentrations at the marginal area of the crown and dentine, even with alveolar bone resorption. However, the amount of bone resorption has a great influence on the stress distribution around the tip of the post compared to the type of crown material.

Evaluation of composite resin core with prefabricated polyetheretherketone post on fracture resistance in the case of flared root canals.

The aim of this study was to use polyetheretherketone (PEEK) for the post material and evaluate the fracture load of six restoration patterns in teeth with flared root canals; composite resin core alone (Group R); glass fiber sleeve (Group S); PEEK post (Group P); glass fiber post (Group F); PEEK post in glass fiber sleeve (Group PS); glass fiber post in glass fiber sleeve (Group FS). In this study, cylindroid specimens were prepared and underwent three-point-bending test in a steady condition and after water immersion. In the loading test, the materials in clinical conditions using bovine teeth were evaluated. In the bending test, groups using glass fiber posts and sleeves decreased in strength after water immersion. In the loading test, Groups F, FS and PS showed higher fracture load than other groups. This study showed PEEK posts and glass fiber sleeves are recommended in the case of flared root canals.

Evaluation of the occlusal contact of crowns fabricated with the bite impression method.

In prosthodontic treatment, reconstruction of a proper occlusal contact relationship is very important as well as reconstruction of a proper interproximal relationship and marginal fitness. Unfortunately, occlusal relationships are sometimes lost in the process of occlusal adjustment of crowns. The purpose of this study was to compare the occlusal contacts of single crown fabricated by two different types of impression techniques. Nine subjects, whose molars required treatment with crown restoration, were enrolled in this study. Full cast crowns were fabricated using two types of impression techniques: the conventional impression method (CIM) and the bite impression method (BIM). The occlusal contacts of crowns were precisely evaluated at the following stages: after occlusal adjustment on the articulator (Step 0), before occlusal adjustment in the mouth (Step 1), after occlusal adjustment at the intercuspal position (Step 2), and after occlusal adjustment during lateral and protrusive excursions (Step 3). The number of occlusal contacts of the crowns on the functional cusps fabricated with BIM was significantly greater than that with CIM after occlusal adjustment. For this reason, the crowns fabricated with BIM might have a more functionally desirable occlusal surface compared to the crowns fabricated with CIM.

Influence of the flexural modulus of prefabricated and experimental posts on the fracture strength and failure mode of composite resin cores.

The purpose of this study was to evaluate the influence of flexural modulus of non-metal posts on the fracture strength and failure mode of composite resin cores. Thirty-two human premolars were divided into four groups: prefabricated glass fiber post with 1.0 mm diameter and composite resin core (Group PE1.0), that with 1.5 mm diameter and composite resin core (Group PE1.5), experimental post (flexural modulus; 4.25GPa) and composite resin core (Group EX), and preparation for abutment teeth only (Group NT). After a static loading test, the fracture strength and failure mode were recorded. Group NT (1670.81 N) showed higher fracture strength than Groups PE1.0 (866.44 N), PE1.5 (825.19 N) and EX (1075.63 N) (p<0.05). This study showed that the fracture strength of endodontically treated teeth restored with composite resin cores is not influenced by the flexural modulus of non-metal posts.

Photoelastic stress analysis of endodontically treated teeth restored with different post systems: normal and alveolar bone resorption cases.

The present study examined the influence of different post materials and their lengths on the mechanical stress of endodontically treated incisor roots in two alveolar bone conditions. Two-dimensional photoelastic models were fabricated to simulate the endodontically treated maxillary central incisors restored with three kinds of posts materials (low Young's modulus glass fiber post, high Young's modulus glass fiber post, and prefabricated stainless steel post) and two post lengths (8 and 4 mm). Completed models were placed in a transmission polariscope and loaded with a static force of 150 N at 45° to the tooth axis. Photoelastic photographs and the magnitudes of fringe order revealed stress distribution in the root, and suggest that the glass fiber post with a low Young's modulus and long length can reduce the stress concentration both in normal and alveolar bone resorption conditions.

The main occluding area in patients with fixed partial dentures.

It has become clear that food crushing is performed in the circumscribed region between functional cusps, that is, at "the main occluding area". However, the position of the main occluding area in patients with fixed partial dentures (FPDs) has not been investigated quantitatively yet. There is a possibility that the load onto the abutment teeth of FPDs is loaded excessively depending on the position of the main occluding area at the early stage of mastication. Therefore the position of the main occluding areas of the FPDs was investigated. The purpose of this study was to evaluate the position of the main occluding area in patients with FPDs quantitatively. We compared the position of the main occluding area between the control subjects who had complete dentition and the patients who wore FPDs for replacing missing first lower molars. Pieces of temporary filling material were used as test material to decide the position of the main occluding area. The centroid coordinate of the test material was calculated and projected onto the reference axis in order to evaluate the position of the main occluding area. Moreover, we recorded the occlusal contact areas at the intercuspal position in order to evaluate the relationship between the position of main occluding area and occlusal contacts.As a result, the position of the main occluding areas in the FPD group were significantly posterior to those in the control group and tended to be less stable. Besides occlusal contact area, existence of periodontal ligaments in the pontic region may cause the difference of the positions of the main occluding area between the FPD group and the control group.

Effect of reinforcement with resin composite on fracture strength of structurally compromised roots.

This study was aimed at evaluating the fracture resistance of structurally compromised roots restored with four different post and core systems. Thirty-two bovine roots were uniformly shaped to simulate human mandibular premolar roots. The roots were divided into four groups based on the type of restoration: cemented cast post and core (Group MC), resin composite build-up (Group CR), resin composite and prefabricated glass fiber post build-up (Group FRC), and thick-layer dual-cured resin composite-reinforced small-diameter tapered cast post and core (Group CRM). After a static loading test, the failure mode and fracture resistance were recorded. Group CRM (719.38+/-196.73 N) exhibited a significantly high fracture resistance compared with the other groups (Group MC: 429.56+/-82.43 N; Group CR: 349.56+/-66.21 N; Group FRC: 398.94+/-112.71 N; p<0.05). In conclusion, Group CRM exhibited better mechanical properties for structurally compromised roots with no ferrules, although all types of restorations showed non-restorable fracture modes.

Investigation of distortions around the cervical area of teeth restored with two kinds of crown materials.

The purpose of this study was to identify crown materials to decrease the stress concentrated at the cervical area of endodontically treated teeth. To this end, 14 extracted human mandibular premolars were divided into two groups for this study: complete cast crowns versus polymer-based crown and bridge material crowns. Both complete cast crowns (MC) and polymer-based crown and bridge material crowns (HC) were cemented with a glycidyl methacrylate-based resin cement (RC) to composite resin cores with glass fiber posts. Static loading was applied and distortion was measured with four pieces of strain gages attached to the marginal area. Findings showed that there was a large difference in distortion between crown and root in MC. On the other hand, distortions at the cervical area of crown and root were similar in HC.

Photoelastic stress analysis of different post and core restoration methods.

The aim of this study was to compare three types of post and core systems by analyzing the stress magnitude within the root. To this end, two-dimensional photoelastic simulation models of endodontically treated maxillary central incisors were fabricated. Three different types of post and core systems were selected for this study: composite resin post and core, composite resin core in combination with a glass fiber post, and conventional cast metal post and core. The fabricated models were observed in a transmission polariscope with the same loading force (400 N) on 45 degrees palatal direction and the fringe orders registered were thereby analyzed. Results obtained in this study suggested that abutment build-up using composite resin core in combination with a glass fiber post model produced the lowest stress concentration and is hence effective in preventing stress concentration in the case of restored endodontically treated teeth.

Relationship between upper and lower molars during mastication.

OBJECTIVES: This study investigated the relationship between upper and lower molars in the early stage of mastication. MATERIALS AND METHODS: We measured the occlusal contact areas during chewing test foods using the three-dimensional shape measurement system and the six-degree-of-freedom mandibular tracking device. The reference points corresponding to "A" and "B" occlusal contacts were selected from the surface of upper dental cast (UA, UB) and lower dental cast (LA, LB). The distance between reference point UA and LA, UB and LB were calculated (/A/, /B/). For evaluation of relationship between upper and lower molars during chewing gum, the ratios of /A/ / /B/ (AB ratio) were calculated. First 10 consecutive cycles of chewing gum were divided into two phase, early five strokes group (Ge) and last five strokes group (GI). Paired t-test for AB ratios of Ge and GI was performed. RESULTS: Paired t-test showed a significant difference between AB ratios of Ge and that of GI at 0.5 mm away from the terminal position of masticatory movements (p < 0.05). CONCLUSION: This result suggests the possibility that the path of mastication is influenced by the change of texture of foods with the progress of mastication in early stage of mastication.

Stress distribution in roots restored with different types of post systems with composite resin.

Although composite resin core is used with various types of prefabricated posts, it remains unclear which kind of material is most suitable for the post. The aim of this study was to evaluate the influence of prefabricated posts on the stress distribution within the root by finite element analysis. Posts and cores were built up with composite resin and four types of prefabricated posts: two types of glass fiber posts (GFP1, GFP2) with low and high Young's moduli, a titanium post (TIP), and a stainless steel post (SSP). In all models, stress distribution during function was calculated. There were differences in stress concentration at the root around the end of posts. The magnitudes of stress for GFP1, GFP2, TIP, and STP were 8.7, 9.3, 11.7, and 13.9 MPa respectively. Given the results obtained, GFP1 was the most suitable material for post fabrication since this model showed a lower stress value. It would therefore mean a lower possibility of root fracture.

Investigation of stress distribution in roots restored with different crown materials and luting agents.

The purpose of this study was to identify crown materials and luting agents that would decrease the stress concentrated at the roots of endodontically treated teeth. To this end, natural tooth model (NT), full cast crown model (gold-silver-palladium alloy; MC), polymer-based restorative material crown model (HCC), and all-ceramic crown model (ACC) were constructed. In each model, methyl methacrylate-based resin cement (MMA) and composite cement (CC) were used as luting agents. The magnitudes of von Mises stress of the roots during function were compared. When the luting agent was changed from MMA to CC, von Mises stress in the cervical area decreased by 37.8% for MC, 27.1% for HCC, and 37.0% for ACC. Within the limitations of this study, the combination of HCC and CC gave rise to the lowest stress concentration at the cervical area.

Comparison of the main occluding area between adults and adolescents.

It has become clear that foods crushing is performed in the circumscribed region between functional cusps, that is, at "the main occluding area". However, it is not established to evaluate this area objectively. The purpose of this study was to establish the positional evaluation of the main occluding area quantitatively and evaluate the positional change depending on the age with this method. The subjects were fifteen adults and ten adolescents who had no stomatognathic disorder. Temporary Stopping was used as test material in this study. In order to analyze the position of the main occluding area, the projected centroid coordinate of the test material was calculated and projected onto the reference axis. It was possible to quantitatively evaluate positional changes of the main occluding area depending on growth process. As a result, the main occluding areas of the adults were significantly posterior to those of the adolescents and tended to be more stable. There was a significant difference in the occlusal contant area between two groups. The occlusal contact area had great influence on the stability of the main occluding area. However, besides occlusal contact area, there might be other factors which stabilized the main occluding area.

Accuracy of temporary laser welding of FPDs by Nd:YAG laser in the oral cavity.

The purpose of this study was to investigate the accuracy of temporary fixation with laser welding for fixed partial dentures (FPDs). Five kinds of experimental FPD with different welding/soldering gaps were fabricated (0, 20, 50 microm for welding; 300 microm for soldering). Then, FPDs were temporary-fixed by laser welding or with a self-curing resin. Fixation accuracy was evaluated by the change in distance and the angular deformation between two retainers. The change in distance and the angular deformation between two retainers of the FPD without welding/soldering gap were significantly larger than the other FPDs (p<0.05). With due consideration to the displacement of teeth or implants especially in the mesiodistal direction, and by taking into account the inevitable errors of the indirect method, it seemed reasonable to provide a welding space of approximately 20 microm.

Study on the fracture strength of root reconstructed with post and core: alveolar bone resorbed case.

This study evaluated the influence of alveolar bone level on the fracture resistance of root restored with post and core. Forty-eight extracted human mandibular premolars were divided into six groups. Cast posts and cores were cemented (MN8 and MP8) or resin cores were built up with fiber posts and composite resin (FN8, FP8, FN4, and FP4). Post length was 8 mm (MN8, MP8, FN8, and FP8) or 4 mm (FN4 and FP4). Specimens were embedded 2 mm (MN8, FN8 and FN4) or 5 mm (MP8, FP8 and FP4) below cement-enamel junction. All specimens were loaded at 45 degrees to the long axis until fracture. With normal bone model, cast post and core (MN8) showed the highest fracture resistance (2262.4 N). However, in the resorbed bone model, there were no significant differences in fracture resistance between cast post and core and fiber post with composite resin.

The effect of occlusal contacts on adjacent tooth.

PURPOSE: The purpose of this study was to clarify the influence of changes of occlusal contacts on adjacent tooth displacement during articulation. MATERIALS AND METHODS: The amount and direction of displacement of the maxillary left first molar and adjacent teeth were measured using a displacement transducer, Type M-3. Seven subjects were included in this study. On the maxillary left first molar, 8 experimental occlusal contact points were established using platinum foil and these were divided into 2 groups, the buccal and lingual groups, which corresponded to the buccal and lingual cusps, respectively. RESULTS: During clenching, the buccal group showed differences in the direction of the tooth displacement paths among the 3 teeth (second premolar, first and second molar), but there was no difference in the lingual group and the direction of displacement of these 3 teeth was similar to the natural teeth. CONCLUSION: It is necessary to make at least one lingual occlusal contact point to obtain displacement of not only the abutment tooth but also the adjacent teeth.

[A statistical survey of the fabrication of single restorations (in 2002)].

A statistical survey based on laboratory records was carried out to determine the state of fabrication of restorations at the dental hospital of Tokyo Medical and Dental University during the six month period from April 1 to September 30, 2002. A comparison was also done with a previous survey that had been carried out in 1986 and 1997 to detect the possible change and alternations in the near future. From the findings of this survey, the following conclusions can be drawn. 1. A total of 7,864 restorations were fabricated during the six months in 2002, and of these, 6,740 restorations (85.7%) were covered by health insurance, and 1,124 (14.3%) were not. 2. Compared with the previous data in 1986 and 1997, decreased numbers of inlays and cast post & cores were the distinctive features that affected the decrease of total numbers. 3. The rate of restorations that were not covered by health insurance slightly increased to 14.3%, compared with 12.3% in 1997.

[Statistical analysis of the fabrication of fixed and removable partial dentures (in 2002)].

A statistical survey based on laboratory records was performed on the number of fixed partial dentures and removable partial dentures fabricated at the dental hospital of Tokyo Medical and Dental University from April 1 to September 30, 2002. A comparison was also performed with a previous survey that had been carried out in 1997 to detect any change and possible alternations in the near future. From the findings of this statistical survey, the conclusions were as follows: 1. A total of 2,656 fixed and removable partial dentures were fabricated during the six-month period in 2002. 2. The 3-unit fixed partial denture was most common (64.0%), and the number of fixed partial dentures decreased as the number of units increased. 3. For a single missing tooth, a fixed partial denture was more popular (80.8%) than a removable partial denture. 4. For two missing teeth, there was little difference between the number of fixed and removable partial dentures. In the anterior region, fixed partial denture was more common, while in contrast, in the posterior region, removable partial denture was more common.

Influence of occlusal height for an implant prosthesis on the periodontal tissues of the antagonist.

The purpose of this study was to investigate a suitable occlusal height for an implant prosthesis by examining the responses of the periodontal tissues around a natural antagonist. The subjects were three Japanese females with two posterior missing teeth restored by ITI system implants (Straumann). Two kinds of experimental implant prostheses were adjusted as follows; one was adjusted in heavy clenching (HC), and the other was adjusted in light clenching (LC). The periodontal pulsation, displacement during biting an occlusal force meter and the mobility of the antagonist were measured before and one week after temporary cementing HC and LC, and one week after removal. In each prosthesis, there was no significant difference in the amounts of the pulsation and mobility of the antagonist before and one week after cementing, and one week after removal (p>0.05). The displacement of the antagonist during biting the occlusal force meter did not change much during the conditions. The results of this study suggested that an implant prosthesis adjusted not only under heavy clenching but light clenching like crown restorations for natural teeth did not affect the periodontal tissues of the antagonist in a harmful manner.